Solvent extraction process

ABSTRACT

Solvent extraction of a solid material which is contaminated with at least one hydrocarbon and water which comprises, in a first extraction stage consisting of one or more extraction steps, extracting a contaminated solid material with a mixture of an alcohol and a hydrocarbon with which it forms an azeotrope, the alcohol and azeotrope forming hydrocarbon respectively being employed at a ratio by volume within the range 30:70 to 70:30. Preferably in a second extraction stage consisting of one or more extraction steps, following the first extraction stage, the treated solid material from the first stage is extracted with a solvent comprising or consisting of the alcohol, thereafter recovering the treated solid material. Suitably the azeotrope-forming hydrocarbon is normal hexane. Suitably the alcohol is ethanol, propanol, isopropanol or a butanol. Suitably the process of the invention is applied to a crude or partially refined product of the growth of a micro-organism on a hydrocarbon substrate in the presence of an aqueous nutrient medium. Preferably the micro-organism is a yeast.

United States Patent Bernard Maurice Laine; Jean Claude Hondermarck;Robert Goux, all of Lavera, Bouches du Rhone, France [72] Inventors [21]App]. No. 700,017

[22] Filed Jan. 24, 1968 [45] Patented Oct. 26, 1971 [73] Assignee TheBritish Petroleum Company Limited London, England [32] Priority Jan. 31,1967 [33] Great Britain [54] SOLVENT EXTRACTION PROCESS 6 Claims, NoDrawings [52] US. Cl 195/28, 195/82 [51] lnt.Cl Cl2c 11/00 [50] Field ofSearch... 195/82, 3 H, 28

[56] References Cited UNITED STATES PATENTS 3,268,419 8/1966 Champagnatet al. 195/82 3,268,412 8/1966 Champagnat et a1. 195/3 PrimaryExaminer-A. Louis Monacell Assistant Examiner-Seymour RandAttorney-Morgan, Finnegan, Durham & Pine ABSTRACT: Solvent extraction ofa solid material which is contaminated with at least one hydrocarbon andwater which comprises, in a first extraction stage consisting of one ormore extraction steps, extracting a contaminated solid material with amixture of an alcohol and a hydrocarbon with which it forms anazeotrope, the alcohol and azeotrope forming hydrocarbon respectivelybeing employed at a ratio by volume within the range 30:70 to 70:30.

Preferably in a second extraction stage: consisting of one or moreextraction steps, following the first extraction stage, the treatedsolid material from the first stage is extracted with a solventcomprising or consisting of the alcohol, thereafter recovering thetreated solid material.

Suitably the azeotrope-forming hydrocarbon is normal hexane. Suitablythe alcohol is ethanol, propanol, isopropanol or a butanol.

Suitably the process of the invention is applied to a crude or partiallyrefined product of the growth of a micro-organism on a hydrocarbonsubstrate in the presence of an aqueous nutrient medium.

Preferably the micro-organism is a yeast.

SOLVENT EXTRACTION PROCESS This invention relates to a process for thesolvent extraction of a solid material which is contaminated with atleast one hydrocarbon and water for the partial to complete removal ofthe contaminants.

According to the present invention there is provided a process whichcomprises, in a first extraction stage consisting of .one or moreextraction steps, extracting a contaminated solid material with amixture of an alcohol and a hydrocarbon with which it forms anazeotrope, hereinafter referred to as the azeotrope-forming hydrocarbon,said alcohol and azeotrope-forming hydrocarbon respectively beingemployed at a ratio by volume within the range 30:70 to 70:30.

Preferably in a second extraction stage consisting of one or moreextraction steps, following said first extraction stage the treatedsolid material from the first stage is extracted with a solventcomprising or consisting of said alcohol, thereafter recovering thetreated solid material.

Preferably the extract fractions recovered from the extraction stagesare fed, separately or after blending, to a distillation stage,consisting of one or more distillation steps, for the separate recoveryof (a) an azeotropic mixture of the alcohol andthe azeotrope-forminghydrocarbon, (b) an azeotropic mixture of the alcohol and water and (c)a residue fraction, thereafter blending part of the azeotropic mixtureof the alcohol and water with part of the azeotropic mixture of thealcohol and azeotrope-forming hydrocarbon, the parts being selected togive a mixture of the alcohol and azeotrope-forming hydrocarboncontaining these materials respectively at a ratio by volume in therange 30:70 to 70:30 and recycling this mixture to the first extractionstage.

Suitably the temperature of the extraction steps lies in the range 30l00C. and where appropriate 3060 C.

Suitably the azeotrope-forming hydrocarbon is normal hexane. Suitablythe alcohol is ethanol, propanol, isopropanol or a butanol.

Suitably the process of the invention is applied to a crude or partiallyrefined product of the growth of a micro-organism on a hydrocarbonsubstrate in the presence of an aqueous nutrient medium.

Preferably the micro-organism is a yeast. Preferably the yeast containsat least percent by weight, more particularly 100-200 percent by weightof water, based on the weight of the pure micro-organism in the drystate. By the term microorganism in the dry state" we mean amicro-organism in the state obtained by drying at 120 C.

If necessary the yeast may be mixed with water before extraction.

Preferably the ratio of water to total alcohol and azeotropeforminghydrocarbon lies in the range 1:4 to 1:10 by weight.

If desired, the extraction as hereinbefore described may be repeated,preferably after addition of water to the yeast to give a water contentas in the first stage.

According to another object of the present invention there is provided aprocess which comprises cultivating a microorganism in the presence of afeedstock which consists of or contains a hydrocarbon, in the presenceof an aqueous nutrient medium and in the presence of a gas containingfree oxygen and thereafter separating part of the aqueous nutrientmedium; thereafter, with or without an intervening treatment, treating amicro-organism-containing product fraction by solvent extraction ashereinbefore described.

Usually the straight chain hydrocarbons will be present in the feedstockaccording to the invention as paraffins: however, the straight chainhydrocarbons may be present as olefins; also there may be used a mixturecontaining straight chain paraffins and olefins;

Suitable feedstoclcs to the process of the invention include kerosene,gas oils and lubricating oils; these feedstocks may be unrefined or mayhave undergone some refinery treatment, but must contain a proportion ofstraight chain hydrocarbons in order to fulfil the purpose of thisinvention. Suitably the petroleum fraction will contain 13-45 percent byweight of straight chain hydrocarbons.

The process of the invention is of particular value for the treatment ofpetroleum gas oil fractions which contain straight chain hydrocarbons inthe form of waxes, sinceby the process of the invention a gas oil ofimproved pour point is obtained while the waxes are converted to avaluable product.

It is an important feature of this invention that when cultivatingyeasts in the presence of the feedstocks hereinbeforedescribed underconditions favoring the growth of the yeasts at the expense of thestraight chain hydrocarbons, the other hydrocarbons, for exampleisoparaffins naphthenes and aromatics are not metabolized or, at most,the proportion which is metabolized is very small. Furthermore, unlikeconventional chemical processes governed by the law of mass action, therate of removal of straight chain hydrocarbons is not substantiallyreduced as the proportion of these hydrocarbons in the overall mixtureof hydrocarbons decreases (except, of course, in the very final stagesof removal). Thus, when desired,.the percentage conversion of straightchain hydrocarbons which is achieved can be maintained at a valueapproaching percent without necessitating a very disproportionateexpenditure of contact time to achieve small improvements. Furthermore,in a continuous process, this high percentage conversion can be achievedwithout resorting to the use of a long reaction path.

By the application of this process under conditions which limit themetabolization of the straight chain hydrocarbons it is possible tooperate with the removal of only a desired proportion of thesehydrocarbons.

Within the term micro-organism used herein we include mixtures ofmicro-organisms. Preferably the micro-organism is capable of growing onat least some normal parafi'msv Micro-organisms which are cultivated asherein described may be yeasts, moldsior bacteria.

The yeasts in this specification are classified according to theclassification system outlined in The yeasts, a Taxonomic Study" by J.Lodder and W. J. W. Kroger-Van Rij, published by North HollandlublishingCo. (Amsterdam) (1952 The bacteria mentioned in this specification areclassified according to the classification system outlined in BergeysManual of Determinative Bacteriology" by R. S. Breed, E. G. D. MurrayandN. R. Smith, published by Bailliere, Tindall and Cox (London) 7thEdition 1957).

Preferably when a yeast is employed this is of the familyCryptococcaceae and particularly of the subfamily Cryp' tococcoideaehowever, if desired there may be used, for example, ascosporogeneousyeasts of the subfamily Saccharomycoideae. Preferred genera of theCryptococcoideae subfamily are Torulapsis (also known as Torula) andCandida. Preferred strains of yeast are as follows. ln particular it ispreferred to use the specific stock of indicated Baarn reference number;these reference numbers refer to CBS stock held by the Centraal Bureauvor Schimmelculture, Baarn, Holland and to INR-A stock held by thelnstitut National de la Recherche Agronomique, Paris, France.

Preferred strain Candida CBS lipvlyll'cn Candida pulchurima CBS 610Candida uu'lt':

Candida un'lis, Variali major CBS 841 Candida lmpicnli: CBS Z3l7Tarulopri: colliculosa CBS l3] Hansenula anomala CBS 1 l0 Oidium IactisNeurorpam silophila Mycodzrma cnncoillole INRA: STV 11 0f the aboveCandida lipolytica is particularly preferred.

if desired the micro-organism may be a mold. Suitable molds arePenicillium and preferably there is used Penicillium expansum. Anothersuitable genus is Aspergillus.

if desired the micro-organism may be a bacterium.

Suitably the bacteria are of one of the orders:

Pseudomonadales, Eubacteriales and Actinomycetales.

Preferably the bacteria which are employed are of the familiesCorynebacteriaceae, Micrococcaceae, Achromobacteraceae,Actincymycetaceae, Rhizobiaceae, Bacillaceae and Pseudomonadaceae,Preferred species are Bacillus megaterium, Bacillus subtilis andPseudamonas aeruginasa. Other strains which may be employed include:

Bacillus amylobacter Pseudomonas natriegens Arthrobacter sp.

Micrococcus sp.

Corynebacterium sp.

Pseudomonas syringae X an lhomonas begoniae Flavobacterium devoransAcelobacter sp.

Actinomyces sp.

Nocardia apaca These bacteria grow in the presence of the followingaqueous nutrient medium:

NH,CI 0.5 grams NaCL 4.0 grams M so,-7i-|,o 0.5 grams Na,HPO -IZH,O 0.5grams KHJO 0.5 grams Water to make up to 1,000 mls.

Preferably the pH of this medium is maintained at 7. Another aqueousnutrient medium is:

K,HPO l gram KH,PO 0.5 grams MgSO JH O 0.5 grams CaCl 0.1 grams NaCl 0.lgrams Water to make up to L000 mls.

A suitable nutrient medium for yeasts and molds has the composition:

(NH,),HPO 2 grams KCl l.l5 grams MgSO -H,O 065 grams AnSO 0.17 gramsMnSOptl-LO 0045 rams FeSO,-7H,O 0068 grams Tap water 200 mls. Yeastextract 0.025 grams Distilled water (to make up to 1,000 mls.)

The growth of the micro-organism used is favored by the addition to theculture medium of a very small proportion of extract of yeast (anindustrial product rich in essential nutrilites, that is, growth factorsobtained by the hydrolysis of a yeast) or more generally of theessential nutrilites. The essential nutrilites include biotin,pantothenic acid, nicotinic acid, thiamine, inositol, and pyridoxine.The quantity of yeast extract added is preferably of the order of 25parts per million. The quantity of each nutrilite required variesbetween about 0.1 parts per million for biotin to about parts permillion for inositol.

The growth of the micro-organism takes place at the expense of thefeedstock fraction with the intermediate production of bodies having anacid function, principally fatty acids, in such a manner that the pH ofthe aqueous mineral medium progressively diminishes. If one does notcorrect it, the growth is fairly rapidly arrested and the concentrationof the microorganism in the medium, or cellular density, no longerincreases so that there is reached a so-called stationary phase.

Preferably therefore the aqueous nutrient medium is maintained at adesired pH by the stepwise or continuous addition of an aqueous mediumof high pH value. Usually, when using molds or yeasts and in particularwhen using Candida lipolytica, the pH of the nutrient medium will bemaintained in the range 3-6 and preferably in the range 4-5. (Bacteriarequire a higher pH usually 6.5-8). Suitable alkaline materials lit foraddition to the growth mixture include sodium hydroxide, potassiumhydroxide, disodium hydrogen phosphate and ammonia, either free or inaqueous solution.

The optimum temperature of the growth mixture will vary according to thetype of micro-organism employed and will usually lie in the range 2535C. When using Candida lipolytica the preferred temperature range is 2832 C.

The takeup of oxygen is essential for the growth of the micro-organism.The oxygen will usually be provided as air. In order to maintain a rapidrate of growth, the air, used to provide oxygen, should be present inthe form of fine bubbles under the action of stirring. The air may beintroduced through a sintered surface. However there may be used thesystem of intimate aeration known as "vortex aeration.

It has been found that by the use of yeast of the strain Candidalipalytica in a process according to the invention in which aeration iseffected by vortex aeration," a high growth rate is achieved whereby thegeneration time lies in the range 25 hours and the cell concentration isincreased by a factor of up to 1,000 in 2 days.

Micro-organisms, and in particular yeasts, when first cultivated withthe use of hydrocarbon fractions as feedstock, sometimes grow withdifiiculty and it is sometimes necessary to use an inoculum ofmicro-organism which has previously been adapted for growth on thehydrocarbon fraction which it is intended to use. Furthermore themicro-organism although cultivated in the presence of an aqueous mineralmedium containing the appropriate nutrient elements, may grow withdifficulty, because the hydrocarbon fraction does not contain the growthfactors which exist in carbohydrate feedstocks, unless these growthfactors are added.

In batch operation, the micro-organism will usually grow initially at alow rate of increase in cellular density. (This period of growth isreferred to as the lag phase"). Subsequently the rate of growth willincrease to a higher rate of growth; the period at the higher rate ofgrowth is referred to as the exponential phase" and subsequently againthe cellular density will become constant (the stationary phase).

A supply of the micro-organism for starting the next batch willpreferably be removed before the termination of the exponential phase.

The growth operation will usually be discontinued before the stationaryphase.

At this stage it will usually be possible to separate the microorganism,contaminated with some unmetalabolized feedstock and aqueous nutrientmedium, from the bulk of the unmetalabolized feedstock fraction.Preferably the separation is achieved by means of a decantation;additionally or alternatively centrifuging may be used. The fractioncontaining a micro-organism is now subjected to treatment with anaqueous-treating medium containing a surface-active agent.

Preferably the micro-organism fraction is vigorously mixed with theaqueous surface-active agent, and, without a further period of growth ofthe micro-organism, is subjected to further separation, preferably bycentrifuging, to recover a micro-organism fraction and a spent aqueousphase containing hydrocarbon impurities removed from the micro-organism.If necessary, the washing and separating steps may be repeated, once ormore, using an aqueous surface-active agent in the washing stage. Afterwashing with surface-active agent it is necessary to wash with anaqueous medium which is free of surface-active agent; preferably thismedium will be water. Again if desired, a series of washing andseparation stages may be employed.

Preferably the washing stages are carried out until the hydrocarboncontent of the micro-organism is less than 7 percent based on the weightof the micro-organism (as calculated for the dry state). Preferably saidcontent of hydrocarbons will be less than 5 percent.

As the surface-active agent employed for washing there may be usedcationic surface-active agents such as stearyltrimethyl ammoniumchloride, nonionic surface-active agents, for example the condensates ofoleic acid and ethylene oxide, or

anionic surface-active agents, for example sodium alkyl sulfates.

The fraction containing the micro-organism is then subjected to solventextraction under the conditions hereinbefore described.

The hydrocarbons recovered in the extract phase by solvent extraction,if metabolizable, may be recycled to the micro-organism cultivationstage.

A yeast which has been freed from the whole or part of its lipids andthe contaminating hydrocarbons by one of the methods describedhereinbefore is a new industrial product.

According to a preferred feature of this invention there is provided aprocess which comprises cultivating a micro-organism in a manner ashereinbefore described in the presence of a petroleum fractionconsisting in part of straight chain hydrocarbons and having a meanmolecular weight corresponding to at least carbon atoms per molecule,and in the presence of an aqueous nutrient medium; and in the presenceof a gascontaining free oxygen, and separating from the mixture, on theone hand, the micro-organism and, on the other hand, a petroleumfraction having a reduced proportion of straight chain hydrocarbons orwhich is free of said straight chain hydrocarbons and thereaftertreating the micro-organism as hereinbefore described.

Preferred methods for use in the cultivation of the micro-organism andfor the recovery of the product are described in British Pat.specification Nos. 914567 and 914568 and US. Pat. application Ser. No.755,019.

This invention is illustrated but in no way limited with reference tothe following examples:

EXAMPLE l 10 liters of the following aqueous mineral medium wasintroduced into a l5-liter stirred fermenter; parts are by weight:

Sodium hosphate, tribasic 3.4 Potassium chloride 0.6 Magnesium sulfate0.3 Ammonium sulfate 2.5

Made up to 1,000 parts with soft water containing trace ele ments. Asuitable alternative medium has the composition:

Diammonium hosphate 2 Potassium chloride l.l5

Magnesium sulfate, 71%,0 0.65 Zinc sulfate 0.l7 Manganese sulfate, "1,00.045 Ferrous sulfate, 7H,O 0.068 Tap water 200 Yeast extract 0.025

Distilled water (to make up to 1,000 Parts).

To the fermenter was added a few parts per million of yeast extract andthen 50 grams of Candida lipolyti a in the form of an aqueous creamcontaining percent by weight of dry material and then 150 grams of aheavy gas oil of petroleum origin containing 20 percent by weight ofnormal paraffins.

When the culture reached the desired concentration of yeast cells forcontinuous operation, the continuous feed, to the fermenter, of aqueousmineral medium and petroleum oil was started up. The temperature wasmaintained at 30 C. and the pH of the medium was maintained regulated atpH 4 by the addition of aqueous ammonia.

This emulsion was fed to a centrifugal separator from which wererecovered three phases, being, in increasing order of density; (a) anoil phase containing the yeast cells, (b) an aqueous mineral mediumphase (which may contain traces of oil and yeast and (c) a yeast creamcontaining approximately one part of yeast, four parts aqueous mediumand a certain quantity of oil adhering to the yeast cells.

The yeast cream and an aqueous solution of a surface-active agent, werefed continuously to a mixer.

The surface-active agent was used at an aqueous concentration of 0.05percent by volume, two parts by volume of the aqueous solution beingadded to one part by volume of the yeast cream. The surface-active agentwas a material sold under the trade designation Ni 29 and being theproduct obtained by condensing a mixture of lauric alcohol and myristicalcohol with ethylene oxide, the product having an ethylene oxide chainofan average 8.5 units per terminal group.

The mixture so obtained was centrifuged to obtain three fractions inincreasing order of density: (a) an oil phase, (b) an aqueous phasecontaining the surfactant product which was recycled to the mixer and(c) a second yeast cream containing one part by weight of yeast (whichwas still slightly contaminated by the oil) with four parts by weight ofaqueous liquid containing the surfactant.

This second yeast cream was passed with water to a mixer and the mixtureso obtained was centrifuged to obtain: (a) an oil phase, (b) an aqueousphase and (c) a thick yeast cream containing 20 percent by weight ofyeast (estimated as dry yeast) and percent by weight of water and whichcontained only a very small quantity of oil.

After a subsequent water-rinsing followed by centrifuging there wasobtained a yeast product containing 65 percent by weight of water,together trace amounts of hydrocarbon contaminant. By furthercentrifuging there was obtained an impure yeast cream containing circaone parts by weight of dry contaminated yeast per 1.5 parts by weight ofwater.

This yeast cream was then pumped into an extractor which was in the formof a filtering drum which was rotated with its axis horizontal. Asolvent mixture consisting of three parts in volume of isopropyl alcoholand five parts in volume of an azeotropic mixture (of which thecomposition is normal hexane 80 percent by weight and isopropyl alcohol20 percent by weight) was poured into the extractor, the volumes ofsolvent being based on one part by volume of dry (contaminated) yeast.

The mixture was maintained at 80 C. for 20 minutes and then the solventdrawn off, finally under vacuum.

A second extraction step was carried out identical to the first.

In a second extraction stage the solid material which was recovered waswashed with isopropyl alcohol at a rate of 4 volumes for 1 volume of dryyeast at 50 C. for minutes. The remaining solvents and contaminant wereremoved. Finally the yeast product was dried in superheated steam.

The analysis of the former yeast cream which was treated by solventextraction and the final yeast product after solvent extraction is givenin the following table.

Yeast cream Final yeast product Nitrogen by wt.

of dry yeast 9.8 ll Total lipids by wt.

ofdry yeast I! 0.5

Fraction (a) 58.7

lsopropanol 18.7 Normal hexane 40.0 Fraction (b) 2] .8 lsopropnnol 18.9Water 2.9 Fraction (c) l9.5 lwpropanol Water l 7.5 Lipids 2.0

Fractions (a) and (b) were blended, to provide the solvents for the twoextraction stages.

EXAMPLE 2 The process described in example 1 was repeated, the secondyeast cream" as therein described was passed with water to a mixer andthe mixture centrifuged to obtain the phases as described except thatthe yeast cream which was obtained contained percent by weight of yeast(estimated as dry yeast) and 84.4 percent by weight of water and 0.6percent by weight of gas oil.

After spray-drying part of this material the balance was blended back togive a yeast paste containing one part by weight of dry contaminatedyeast per 1.5 parts by weight of water. This material was pumped into anextractor and further treated as described in example i with the sameresult as described therein.

Example 3 The process described in example 1 was repeated except thatCandida lropr'calis was used in place of Candida lipalytica.

Similar results were obtained.

EXAMPLE 4 The process described in example 2 was repeated except thatCandida tropicalis was used in place of Candida lipalytica.

Similar results were obtained.

What we claim is:

l. A process for the purification of a solid containing material whichis a crude or partially refined product of the growth of amicro-organism on a hydrocarbon substrate in the presence of an aqueousnutrient medium, said solid micro-organism containing material beingcontaminated with at least one hydrocarbon and which contains water inan amount in excess of the amount of water present in the micro-organismin the dry state which process comprises, in a first extraction stageconsisting of one or more extraction steps, extracting said solidmicro-organism containing material with a mixture of an alcohol and anazeotrope-forming hydrocarbon, said alcohol and azeotrope-forminghydrocarbon respectively being employed at a ratio by volume within therange 30:70 to 70:30, thereafter in a second extraction stage consistingof one or more extraction steps, following said first extraction stage,the treated solid micro-organism containing material from the firststage is extracted with alcohol as the solvent, and thereafterrecovering the purified micro-organism.

2. A process according to claim I wherein the extract fractionsrecovered from the extraction stages are fed, separately or afterblending, to a distillation stage, consisting of one or moredistillation steps, for the separate recovery of (a) an azeotropicmixture of the alcohol and the azeotrope-forming hydrocarbon, (b) anazeotropic mixture of the alcohol and water and (c) a residue fraction,thereafter blending part of the azeotropic mixture of the alcohol andwater with part of the azeotropic mixture of the alcohol andazeotrope-forming hydrocarbon, the parts being selected to give amixture of the alcohol and azeotrope-forming hydrocarbon containingthese materials respectively at a ratio by volume in the range 30:70 to70:30 and recycling this mixture to the first extraction stage.

3. A process according to claim 1 which the azeotropeforming hydrocarbonis normal hexane.

4. A process according to claim 1 in which the alcohol is ethanol,propanol, iso ropanol or a butanol.

. A process accor mg to claim I which at least one solvent we UNITEDSTATES PATENT OFFICE CERTIFICATE OF CORRECTION Iatent No. 3,616,209Dated October 26, 1971 Inventor) Bernard Maurice Laine, Jean ClaudeHondermarck and Robert CEux It is certified that error appears in theabove-identified potent and that said Letters Patent are herebycorrected as shown below:

...continued...

Column 6, line 26, "parts" should read part Claim 1, line 1, after"solid", insert micro-organism Claim 5, line 1, after "claim 1" insertin and line 3, 30-100bC." should read 30lO0C.

Signed and sealed this 16th day of May 1972.

(SEAL) Attest:

EDWARD M.FLEICHER, JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents Pag 2 of g

2. A process according to claim 1 wherein the extract fractionsrecovered from the extraction stages are fed, separately or afterblending, to a distillation stage, consisting of one or moredistillation steps, for the separate recovery of (a) an azeotropicmixture of the alcohol and the azeotrope-forming hydrocarbon, (b) anazeotropic mixture of the alcohol and water and (c) a residue fraction,thereafter blending part of the azeotropic mixture of the alcohol andwater with part of the azeotropic mixture of the alcohol andazeotrope-forming hydrocarbon, the parts being selected to give amixture of the alcohol and azeotrope-forming hydrocarbon containingthese materials respectively at a ratio by volume in the range 30:70 to70:30 and recycling this mixture to the first extraction stage.
 3. Aprocess according to claim 1 which the azeotrope-forming hydrocarbon isnormal hexane.
 4. A process according to claim 1 in which the alcohol isethanol, propanol, isopropanol or a butanol.
 5. A process according toclaim 1 in which at least one solvent extraction step is carried out ata temperature in the range 30*-100* C.
 6. A process according to claim 1in which the micro-organism contains at least 20 percent of water inexcess of the water present in the micro-organism in the dry state.